The present invention is related to a locking device for safety seat belts in vehicles of the kind in which a seat belt strap is automatically retracted, at least partially, into a housing when the seat belt is not used. The locking device is primarily intended to be used in safety seat belts of the kind in which the strap is retracted into a housing without being wound on a spindle or similar means associated with a reel.
It is previously known to collect the retracted strap of automatically retractable safety seat belts in folds or loops which run more or less freely in a limited space. In this way the use of a rotatable spindle for winding up the strap is avoided, thus facilitating the positioning of the seat belt retracting device in the vehicle, as the space requirement can more easily be met. Due to the increase in the number of possible locations, the advantage is obtained that it often is possible to decrease the required length of the seat belt strap. Furthermore, the advantage is obtained that the force necessary for extraction of the seat belt strap will be substantially constant during the extraction. When the strap is wound on a spindle connected to a helical spring which is tensioned when the strap is extracted and then provides the automatic retraction of the strap, when the seat belt is not used, the force required for the extraction of the strap will increase gradually during the extraction of the strap, because the diameter of the strap reel is gradually decreasing during the extraction. The properties of the spring have a comparatively small influence on the variation in the necessary extraction force and, therefore, this variation is mainly due to the varying diameter of the strap reel. An increase of the extraction force during the extraction of the seat belt is a considerable disadvantage, beacause the pressure from the seat belt can be uncomfortable to the person using the seat belt.
In retracting devices in which the retracted strap is laid in folds or loops, two or more rollers are normally used for driving and locking the strap, the strap running around the rollers and one of the rollers beeing connected to a spring which is tensioned during the extraction of the strap and which then automatically causes the roller to rotate in the opposite direction for retraction of the strap, when the seat belt is not used. Several embodiments are known in which these rollers enable the locking of the strap when the strap is rapidly extracted, for instance in case of a rapid braking of the vehicle, so that a strap-sensitive locking device is provided for the strap. However, these embodiments have been comparatively complicated and, furthermore, have not always provided a sufficently rapid and safe locking of the strap in critical situations. Some locking devices have also caused heavy wearing of the strap when locking, and this means in the long run a decrease in the reliability of the seat belt.